Personally, I have never listened to O&A. I am sure they can be just as entertaining as Stern, but since I have listened to Stern for 10 years now, I am so happy he is on Sirius now. His show has been so much better than the previous 5 years or so. Don't forget, his listeners had never heard the unedited O'Brien clips so it was very funny and entertaining to me.
One important thing to point out is that Stern coming to Satellite radio is good for both XM and Sirius. It keeps the interest in a new technology and it keeps XM on its toes to continue to provide quality programming for its own listeners. Eventhough you may not like Stern, you will benefit from his jump no matter which company you go with.
As I said before, to each his own.